Rollup merge of rust-lang#69645 - DutchGhost:const-forget-tests, r=Dy…

…lan-DPC

const forget tests

Adds tests for rust-lang#69617

src/libcore/tests/lib.rs

@@ -41,6 +41,7 @@
 #![feature(never_type)]
 #![feature(unwrap_infallible)]
 #![feature(leading_trailing_ones)]
+#![feature(const_forget)]
 
 extern crate test;
 

src/libcore/tests/mem.rs

@@ -129,3 +129,21 @@ fn test_discriminant_send_sync() {
     is_send_sync::<Discriminant<Regular>>();
     is_send_sync::<Discriminant<NotSendSync>>();
 }
+
+#[test]
+fn test_const_forget() {
+    const _: () = forget(0i32);
+    const _: () = forget(Vec::<Vec<Box<i32>>>::new());
+
+    // Writing this function signature without const-forget
+    // triggers compiler errors:
+    // 1) That we use a non-const fn inside a const fn
+    // 2) without the forget, it complains about the destructor of Box
+    const fn const_forget_box<T>(x: Box<T>) {
+        forget(x);
+    }
+
+    // Call the forget_box at runtime,
+    // as we can't const-construct a box yet.
+    const_forget_box(Box::new(0i32));
+}